It's obviously not supposed to be, but in the infamous AIDS episode, the nonchalant way the doctor says, "There are only a few ways to contract the virus — using drugs with needles, unprotected sex, or that blood transfusion a few years back..." without a single pause is just... priceless.
And of course, "AIDS stinks!"
In one episode — a real episode of this series, mind you — Captain Planet is sent to jail.
During the pilot, Wheeler accidently activates his ring by merely saying fire, and blasts the floor of the Geo Cruiser with it. A small fire starts, and Gi is forced to rush to extinguish it with her own ring.
Ma-Ti: Wheeler! Don't say "fire" until you mean it!
In one episode, Dr. Blight kidnaps her good twin sister Bambi and prepares to steal her identity:
Dr. Blight: Time to walk a mile in my sister's shoes.
MAL: Her shoes, maybe, but not her jeans, judging by the rate that you are eating those donuts.
(Gilligan Cut to Dr. Blight trying to put on Bambi's jeans and ripping them)
In the same episode, when Dr. Blight pulls a switch on Bambi, she engages in Bad Bad Acting. They say "We'll get Kath Soucie to loop over" and Linka says "I hope they get this Soucie person." Extra funny because of who their voice is.
There's one episode, where in the future where pollution runs rampant and animals are all apparently extinct, the Planeteers, now in their 40s (except the one who was time traveling), have to call out Captain Planet, which starts out normal enough... Until he appears as somewhat obese and balding, and apparently has gotten a bit grouchy over the years.
Captain Planet: With your powers combined-! (hacking cough) Alright, waddaya want?